This invention relates to an illuminating device for photographic copying apparatus comprising a light source, an image-producing optical unit, a light-mixing element having a light entry face provided with parallel ribbings, and a filter unit interposed into the path of light rays from the light source to the light-mixing element, which filter unit comprises a window for the passage of light and serves for regulating the color composition of the light emitted by the light source. Such illuminating devices are used, for instance, in the Color Printer of Type No. 3140 manufactured by Gretag AG, Regensdorf, Switzerland and marketed in the United States by CX Corporation, Seattle, Wash.
With increasing efficiency of automatic copying apparatus, the demands made on the copying light and, together therewith on the illuminating device producing the same, are likewise rising all the time. In particular, increasingly higher light intensities are demanded in order to attain shorter exposure times and correspondingly higher copying speeds.
With increasing light intensities, the difficulties of achieving homogenization of the color distribution across the illuminated surface are likewise augmented. In particular it is difficult to obtain a uniform color distribution independently of the positioning of the filter. Furthermore it becomes necessary to pay increased attention to the degree of light efficiency with the aim of avoiding losses of light as much as possible.